"A buyer would just have no way of being able to know with the information they have whether that milk is safe."

Keim said she and her colleagues had noticed milk sharing websites popping up over the past few years.

For the new study, they responded to 495 ads placed on two of those sites and ended up purchasing and analyzing 101 batches of milk.

Seventy-four percent of the samples either had disease-causing bacteria like E. coli or harmful levels of bacteria such as Streptococci.

In either of those instances, "You just shouldn't feed that raw to a baby," Keim said.

The study did not determine whether all the contaminated milk would make babies sick. But the researchers said it could be risky, particularly for preemies and other infants with higher than average risks for medical problems.

The U.S. Food and Drug Administration recommends against feeding babies milk bought over the Internet, but doesn't regulate its sale.

Researchers said human milk always contains some bacteria, even before it comes out of the breast.

"It's totally normal for milk to have bacteria in it, and there are bacteria in milk that are extremely important for babies to build their immune and digestive systems," Keim told Reuters Health. "Those are not the bacteria that we looked at."

Milk samples that spent more time in transit were more likely to contain potentially harmful bacteria. But sellers' claims that they were healthy or their milk was safe did not predict which samples actually had potentially harmful bacteria.

None of the shipped breast milk tested positive for HIV, the virus that causes AIDS. But one in five samples had traces of cytomegalovirus, which can cause disease in babies with weak immune systems.

The researchers also looked at 20 milk samples that had been donated to a milk bank for comparison. Seven of those contained disease-causing bacteria, according to the findings published Monday in Pediatrics.

Dr. Ekhard Ziegler said he believes it's too early to tell parents to avoid purchasing milk at all costs.

But, he added, "What we need now is someone studying the recipients of Internet milk, (to see) whether there is any indication that they have more illness."

Ziegler was not involved in the new research. He told Reuters Health he has seen a baby who developed sepsis from milk it received from its own mother that was heavily contaminated with bacteria.

Keim and her colleagues chose not to name the websites they used to order milk.

Administrators of one site that facilitates the sale of milk, www.onlythebreast.com http://www.onlythebreast.com, told Reuters Health this weekend that they recognize the problem and are trying to address it.

"We have made the decision to transition away from offering breast milk classified ads and in the near future completely remove them with the goal (of stopping) all...activities related to informal milk sharing," the site's administrators said in a statement.

"We are convinced," they wrote, "that a more safety centered approach must be taken to secure milk sharing. After careful consideration, we will be shifting our focus from mother to mother internet sales to helping qualified Only the Breast donors provide milk to a new milk banking program."

The company says it will screen its donors and process their milk in partnership with Medolac Laboratories.

Keim said women who have trouble breastfeeding should turn to a pediatrician or lactation consultant for support rather than looking elsewhere for human milk.

And women who are making extra milk should know they can donate to a milk bank, where the milk will go directly to premature babies in hospitals, she said.

The World Health Organization recommends exclusive breastfeeding - without any formula or solid food - until a baby is six months old, followed by breastfeeding with the addition of appropriate foods through age two.